1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a compensation film, a manufacturing method thereof and a liquid crystal display using the same.
2. Discussion of the Related Art
According to a recent rapid advance to information society, a flat panel display is required to have an excellent characteristics such as slimness, light weight and low power consumption.
A Liquid Crystal Display (LCD), one of the flat panel displays, has an excellent visibility in comparison with a Cathode Ray Tube (CRT) and has low average power consumption and caloric value in comparison with a CRT having the same screen size. For these reasons, LCDs are used in portable phones, computer monitors, television sets and the like and highlighted as next-generation display together with Plasma Display Panels (PDP) and Field Emission Displays (FED).
Generally, as LCD includes two substrates on which electric-field generation electrodes are formed. The two substrates are arranged to face each other and liquid crystal material is injected between the two substrates. An electric field is induced by a predetermined voltage, which is applied to the electric-field generation electrodes. By controlling liquid crystal molecules according to the electric field, light transmittance is controlled. Through these procedures, images are displayed on the LCD.
Generally, liquid crystal molecules are aligned anisotropically. The liquid crystal molecules have a property such that the anisotropy of the liquid crystal molecules changes depending on a distribution degree of liquid crystal molecules and a distribution degree of tilt angles with respect to the substrate. Accordingly, such a property of the liquid crystal is an important factor that causes a polarization to be changed depending on a viewing angle of the liquid crystal cell or the film.
In driving the LCD panel, such an inherent property of the liquid crystal molecules causes a luminance and a contrast ratio to be changed depending on the omni-directional viewing angles. Therefore, the LCDs have problems that cannot obtain a constant luminance and contrast ratio through a range of viewing angles.
In order to overcome the foregoing problems, a compensation film has been proposed for compensating for anisotropic distribution depending on the viewing angle of the liquid crystal cell.
In the compensation film, a phase difference with respect to a transmitted light is varied by a polymer film. Also, the compensation film extends in a predetermined direction to have birefringence due to anisotropic induction of the molecules.
For example, when an external electric field is applied to a Twisted Nematic (TN) mode liquid crystal display having a normally black mode, the liquid crystal molecules respond to the applied electric field and provide light transmittance like a following equation.I=Io sin2[θ(1+u2)½],U=πR/θλ, R=Δn·d  [Equation]                where,        I: an intensity of a transmitted light        Io, an intensity of an incident light        Δn: a birefringence        d: a thickness of a liquid crystal cell        λ: a wavelength of a transmitted light        θ: a twist angle of a TN liquid crystal        R: a phase difference.        
That is, when light passes through the liquid crystal molecules in a vertical direction and in an oblique direction, values of the phase differences are different from one another. Thus, the properties of the transmitted light are changed depending on the viewing angle.
A birefringence value (Δn·d) of the light passing through the liquid crystal is evaluated by multiplying a difference value of refractive index on a plane perpendicular to a light forwarding direction by a thickness of a medium through which the light passes.
In order to compensate for the phase difference of the liquid crystal, the compensation film with a liquid crystal layer constructed to have a birefringence value almost identical with a birefringence value (d*(ne−no)) of the liquid crystal and to have a negative phase value (ne−no) can be used to compensate the viewing angle.
Accordingly, the viewing angle problem can be solved by compensating for the phase difference in an opposite direction in the compensation film, which is installed between the liquid crystal substrate and the polarizing plate in order to compensate for the phase difference inside the liquid crystals. At this time, a uniaxial film or a biaxial film is used as the compensation film.
FIGS. 1A to 1C are views illustrating a refractive-index anisotropic ellipsoid of a phase-difference compensation film.
As illustrated in FIGS. 1A to 1C, assuming that X-, Y-, and Z-direction refractive indices are respectively expressed as “nx”, “ny” and “nz”, uniaxiality and biaxiality are determined depending on whether or not the X-direction refractive index “nx” is identical with the Y-direction refractive index “ny”. That is, as illustrated in FIG. 1A, the uniaxiality refers to a case where refractive indices of two directions (X- and Y-directions) are equal to each other but different from a refractive index of the remaining direction (Z-direction). Additionally, as shown in FIGS. 1B and 1C, the biaxiality refers to a case where refractive indices of three directions (X-, Y- and Z-directions) are different from one another.
The general phase-difference compensation film using the uniaxial refractive-index anisotropic body is aligned such that a long axes of the ellipsoid are in parallel with and perpendicular with a surface of the phase-difference film.
A conventional manufacturing method of the phase-difference film uses a method of extending a high molecular film uniaxially or biaxially to allow a light axis of the phase-difference film to have a predetermined angle with respect to a film forwarding direction, thereby obtaining a the desired birefringence.
FIG. 2 is a view schematically showing a structure of a conventional LCD with a compensation film.
Referring to FIG. 2, the conventional LCD includes a liquid crystal panel 20 formed by injecting a liquid crystal layer 10 between the upper and lower substrates 11 and 12, in which upper and lower substrates 11 and 12 are spaced apart from each other by a predetermined distance; first and second compensation films 13 and 14 attached to outer surfaces of the upper and lower substrates 11 and 12; and first and second polarizing plates 15 and 16 attached to the compensation films 13 and 14 to have optical transmittance axes perpendicular to each other.
At this time, since the compensation films 13 and 14 have an anisotropic distribution with opposite direction to liquid crystal cells of the liquid crystal layer, it is possible to eliminate a difference of light retardation according to a viewing angle when using the compensation film attached to the liquid crystal cells.
FIG. 3 is a view schematically showing a structure of the compensation film of FIG. 2.
As shown, the compensation film has a refractive index anisotropy by arranging discotic liquid crystals 17 to change tilt angles successively. At this time, the discotic liquid crystals 13 can be formed using a splay alignment.
However, the conventional compensation film provided in the LCD has a limit that a wider viewing angle cannot be obtained because it is difficult to make the discotic liquid crystals have successive tilt angles different from each other.